The invention is directed to a thermal fixing system for fixing toner images on the front side of a web-shaped recording medium in an electrographic printer or copier device, whereby the back side of the recording medium can already have a fixed toner image.
Thermal fixing devices that comprise a pre-heating saddle with a following fixing zone composed of a heated fixing drum and a pressure roller are employed in printer or copier devices for heat transfer fixing of toner images on a recording medium that is usually composed of paper.
Such thermal fixing devices are disclosed, for example, by U.S. Pat. No. 4,147,922 or Japan Abstract Vol. 13, No. 120, 24 Mar. 1989 (Japan-A-63-292177).
It is beneficial in electrographic printer devices that work in the highest speed range with, for example, a printing speed of more than 0.5 m/s, and that employ a heat transfer fixing station for fixing, to heat the paper web or the paper sheet to temperatures of approximately 100.degree. C. or more before the actual heat transfer fixing process in order to thus obtain a good joining of the toner image to the paper surface.
When a paper web or a single sheet of paper that is already printed and fixed on one side, for example on the back side, is to be printed and fixed on the other side, then the first side which is already fixed must be conducted over the hot surface of the pre-heating saddle for heating the paper for the second fixing process. The following problems thereby arise in this second fixing process:
a) Continuous printer operation:
The print image that is already fixed and that runs over the hot surface of the pre-heating saddle is heated to such an extent that it assumes a condition ranging from tacky through fluid, and is partly smeared on the saddle surface. The more toner is transferred from the toner image onto the saddle surface the more toner collects on the saddle surface, until a visible destruction of the toner image on the paper occurs.
b) Waiting or Standby Operation:
While the printer is in the waiting or standby mode, the paper web having the already fixed print image lies on the hot saddle. The print image is heated to such an extent in the region of the surface of the pre-heating saddle that it assumes a tacky through fluid condition and sticks to the hot surface of the pre-heating saddle. When the paper web is started, the toner image is then torn from the surface of the paper web and remains sticking on the hot surface of the saddle.